A recent significant advancement in the field of generating low temperature refrigeration is the pulse tube system or cryocooler wherein pulse energy is converted to refrigeration using an oscillating gas. Such systems can generate refrigeration to very low levels sufficient, for example, to liquefy helium. One important application of the refrigeration generated by such cryocooler system is in magnetic resonance imaging systems.
A pulse tube cryocooler is a hermetically-sealed, constant volume apparatus containing a fixed charge of a working gas, usually helium. To date they have typically been studied in indoor laboratory environments where there is little variation in ambient temperature. As they are commercialized and utilized in outdoor environments, or at least exposed to outdoor temperature patterns, they may experience large temperature fluctuations which could cause significant changes in the internal mean pressure since the cryocooler has constant volume and contains a fixed charge of working fluid. It has not been recognized that these mean pressure fluctuations can severely impact cryocooler performance.
Accordingly, it is an object of this invention to provide a method for operating a pulse tube cryocooler which can improve the performance of the cryocooler when the cryocooler undergoes one or more mean pressure fluctuations.